My experiments suggest that -3 is needed to get the last 2 digits rather than -2; at first I thought it was due to the space before the pipe, but it needs it even without the space there. Perhaps it's the End-Of-Data marker ?

According to the man page for tail, the - before the number is optional ; either will count from the end of the input. If you use + instead it will count from the beginning of the input. I tend to put optional things like that in when I'm writing scripts just to make it really obvious what I mean when I come back to it months or years later